audio policy: remove AudioSession class

Now that clients are tracked individually, tracking by
audio session is no needed anymore.
Modify management of client activity, preempted sound trigger
sessions and silenced uids to rely on RecordClientDescriptor
instead of AudioSession.

Test: CTS test for AudioRecord
Test: manual capture audio tests for all use cases

Change-Id: Ieef3e05daea98aec9320412393f912bd6bd7e046